The use of coatings to provide corrosion protection to an underlying article or substrate is common. Protective coatings can include organic coatings such as paints and epoxies; nonmetallic coatings such as cements, enamels and oxides; and metallic coatings such as chrome and gold plating. Application of these coatings can be accomplished by such differing processes as painting and spraying to plating and vapor deposition. The process of applying the coating is often dependent on or limited by the properties of the material being deposited and of the properties of the substrate.
Research on improving protective coatings has been extensive for many different materials and applications. The object of protective coatings is to provide corrosion resistance to the underlying substrate and enhance the corrosion resistance of the substrate to the various environments the substrate may encounter. Many coatings are limited to particular environments because of their inability to withstand certain temperature and/or corrosive conditions. The use of organic binders in many coatings limits the use of those coatings at elevated temperatures. A coating not requiring organic binders may withstand elevated temperatures.
Additionally, many articles requiring corrosion protection have specific weight limitations. Therefore, thinner and accordingly lighter coatings are desired. Thinner coatings are also desirable because they require less material, do not significantly change the substrate size, and offer the potential to reduce material costs. Protective coatings, regardless of their composition and the manner in which they are applied, must be adherent to the substrate they are to protect. In order to protect the underlying substrate, the protective coatings must act as a protective barrier against the corrosive agent or as a sacrificial layer. Sacrificial protective layers have the disadvantage that a sacrificial protective layer only provides temporary protection and must be replaced once it has been expended.
Inorganic coatings have also been used for corrosion protection. However, inorganic coatings are typically made of materials that have low coefficients of the thermal expansion relative to the higher coefficient of thermal expansion metal substrates they are intended to protect. While inorganic coatings may perform adequately at a particular temperature, the inorganic coatings on the metal substrates are not able to withstand large temperature changes. When the metal substrate and the coating are subject to large temperature increases and decreases, the underlying metal substrate expands and contracts, respectively, to a greater degree than the inorganic coating. The coefficient of expansion mismatch causes the brittle inorganic coating to crack and break away from the surface of the metal, a phenomenon known as spalling. Thus, the metal is no longer protected by the coating and may become exposed to the corrosive agents.
Metals have been used as protective coatings. However, most metals are subject to corrosion, especially at elevated temperatures and in aqueous, salt and acidic environments. Additionally, metal coatings are expensive, heavy and can be removed by abrasion.
Accordingly, there is a need for an improved corrosion-resistant coating which is more durable and effective under a broader range of conditions, particularly at elevated temperatures and in acidic and saline environments.